Conventional metal detecting apparatuses for use on the inspection line of food products or the like use a method by which, so that any metal contained in the object of inspection can be detected while it is being conveyed, a magnetic field is generated on the conveyance route of the object of inspection, variations in the magnetic field due to any metal contained in the object of inspection are detected.
FIG. 19 shows the configuration of a metal detecting apparatus 10 which detects variations in the magnetic field.
This metal detecting apparatus 10 has a signal generator 11 which supplies a signal D of a prescribed frequency, a transmission coil 12 which receives the signal D and generates an alternating field E of a prescribed frequency on the conveyance route 2 of the object of inspection 1, a magnetic field variation detecting unit 13 which, having two reception coils 13a and 13b arranged in the conveying direction of the object of inspection 1 in such positions that they receive equal quantities of the alternating field E and differentially connected to each other, is to detect a signal matching a variation in the magnetic field due to any object passing the alternating field E, a detector unit 16 which synchronously detects the output signal R of the magnetic field variation detecting unit 13 with a signal having the same frequency as the signal D, and a control unit 17 which determines the presence or absence of any metal in the object of inspection 1 on the basis of the output signal of the detector unit 16.
In the conventional metal detecting apparatus 10 configured as described above, as the signals generated on the two reception coils 13a and 13b are in an equilibrium with their amplitudes and their phases reverse between each other when the object of inspection 1 is absent in the alternating field E, the amplitude of the signal R becomes zero, and so does the output of the detector unit 16. On the other hand, when the object of inspection 1 is present in the alternating field E, as the equilibrium between the signals generated on the two reception coils 13a and 13b is disturbed by the influences of the object of inspection 1 itself and any metal contained in the object of inspection 1, and the signal R whose amplitude and phase varies with the movement of the object of inspection 1 is supplied.
The signal R then contains not only a signal component due to the influence of the contained metal on the alternating field E but also a signal component due to the influence of the object of inspection 1 itself (including the packing material or the like) on the alternating field E, and this latter signal component due to the object of inspection 1 itself determines the detectable limit of the contained metal.
This influence of the object of inspection 1 itself on the alternating field significantly differs with the water content, the packing material and other factors of the object of inspection.
For this reason, according to the prior art, the phase of synchronous detection is so set in advance as to minimize the amplitude of the output signal of the detector unit 16 when a good sample of the object of inspection 1 is caused to pass the alternating field E, a voltage greater than that minimum amplitude is set as the threshold either automatically or manually. Or a multiplying factor for the minimum amplitude is manually designated, a voltage multiplied by that designated factor set as the threshold. The object of inspection 1 is inspected according to one of these thresholds and, when the object of inspection 1 has passed the alternating field E, if the amplitude of the output signal of the detector unit 16 surpasses the threshold, the object of inspection 1 will be determined as containing metallic foreign matter.
One of such techniques by which the threshold for the detection of metal is set in terms of a voltage of a multiplying factor is disclosed in the following Patent Reference:
Japanese Patent No. 2574694
However, a method by which by which the threshold for determining the presence or absence of any metal content is entered or designated in terms of a voltage of a multiplying factor does not allow the user to know intuitively how much metal can be detected.
Moreover, the user cannot know how much metal is present in any object of inspection determined to contain metal, making the method inconvenient for the user to rely on.
An object of the present invention, intended to solve this problem, is to provide a metal detecting apparatus which expresses the threshold for determination or any metal content in the object of inspection in its own magnitude, so that the user can intuitively know how much metal is contained and would find it easier to use.